The most commonly used type of memory circuit in a relay protection device is a resonance circuit, active or passive, tuned to the rated frequency. This type of memory circuit has two disadvantages. Firstly it has a poor memory when it comes to remembering for a long period of time, and secondly it has to be pre-set to a rated frequency, which may differ a few percent from the operating frequency of the network just prior to the fault occurring. This difference in frequency affects the operating characteristic of the protection device and may, in the worst case, result in a malfunction. The dependence of the memory circuit on the frequency of the network also makes it necessary to retune the memory circuit upon a change in the rated frequency of the network. This results in increased costs of production. To avoid the problems associated with the frequency dependence of resonant circuits, a phase-locked loop may be used for the memory circuit. The great problem in connection with using a phase-locked loop is to rapidly disconnect the feedback signal when the phase position of the input quantity being used to monitor the supervised network changes, so that the output signal is not affected.